This invention relates to systems and procedures for printing documents. These documents can contain all types of visual information, such as text, graphics, images, etc., further commonly referred to as xe2x80x9cimagexe2x80x9d
A well-known problem that is common to all recording systems, that convert image data into discretised pixel information, is the occurrence of reconstruction errors, commonly called xe2x80x98aliasingxe2x80x99. It results in jagged edges at lines or curves that are not aligned with the discrete pixel grid of the recording apparatus or printer.
This problem can be cured in two ways. The first one is to increase the spatial resolution of the recording device. This is often a technically difficult and costly solution.
A second approach, and this is the approach to which the present invention relates, is to trade spatial resolution with tonal resolution in order to improve the visual quality.
This approach is common for font rendering on computer displays. Here black pixels are surrounded by grey pixels. This results in visual smoothing of the edges, rendering the edges in a way more pleasing to the human eye.
A procedure is disclosed that creates multi-level gradation data from an abstract page description to achieve a similar result. Most documents to be printed contain a mixture of text, graphics and images and are given in the form of an page description. These documents may be PDF documents, PostScript(trademark) files, MS-Office(trademark) documents and so on. We have found that spatial resolution and tonal resolution are to some extent interchangeable and that lack of spatial resolution can be compensated by improved tonal resolution. The use of multiple gradation levels enhances the tonal resolution in portions of the document where the data is highly discretised, such as text, drawings, etc.
This technique requires that the recording engine has a means of rendering more than two different levels of grey or some other color, i.e. more than 0% ink and 100% ink. However most ink-jet and bubble-jet printing devices can only deliver fixed-size drops of ink making the printing of multi-level pixel data without reducing the spatial resolution impossible.
Solutions to this problem have been proposed in terms of using multiple inks of the same color, but different density. We refer to the documents WO 96/12251, EP 0 850 767 A1 and U.S. Pat. No. 5,966,507 herein incorporated by reference. A method of surrounding black pixels by grey pixels for use in a 3-level electrophotographic printer was disclosed in EP 0 082 281 A1 herein incorporated by reference.
Further developments in electrophotographic printing allow the pixel size to be adapted. Procedures to enhance the resolution in that way, working by comparing the neighbourhood of a pixel with pre-existing templates, are described in U.S. Pat. No. 4,847,641, U.S. Pat. No. 5,193,008 and U.S. Pat. No. 5,134,495 herein incorporated by reference.
This invention however differs from the above approaches in that it combines a number of image processing steps, different from the ones described in the above-cited references, with a technology to record multiple gradation levels at a single pixel, with the same ink.
It is an object of the present invention to provide a printing process in which the visual quality of the obtained image is enhanced.
It is a further object to enable the softening of jagged edges at lines or curves that are not aligned with the discrete pixel grid of the recording apparatus or printer.
The above mentioned objects are realised by a method having the specific features set out in claim 1. Specific features for preferred embodiments of the invention are set out in the dependent claims. These objects are also realised by a system having specific features of claim 11.
The above objects are mainly released by combining:
a process for printing multiple gradation levels at a single pixel, with the same ink and
a procedure for obtaining multi-level gradation data from an abstract page description of the document to be printed.
An image as referred to hereinafter can be anything on a document such as text, graphics, images, etc. . . . The page data describes the layout and content of the page to be reproduced, this data can be supplied in various formats.
Rendering an image is the process of reproducing the image or document on a medium or on a display screen.
A bitmap is typically a two-dimensional array of pixels. Each pixel represents a small square or rectangular portion of an image. In grey images, each pixel may be represented by one value e.g. in the range of 0-255. Such a bitmap is called a multi-level bitmap as every pixel in the bitmap may take one of several available density values or gradation levels.
In color images, each pixel is typically represented by three or more color components. For each color component of each pixel a value is required. In a system with three color components, where each color pixel value is represented by 8 bits, each color pixel may take 256 different values for each component. Consequently each pixel may take 2563=16,777,216 possible values. Besides the three color values, extra bitmap values can be calculated for the black color component. The output bitmap is a bitmap which is sent to the printer for rendering of the image by the printer.
The resolution of an image is the number of pixels used to represent a linear size of the image. For a physical representation of an image this is expressed as the number of pixels or dots per unit of length, e.g. 16 dots/mm or 400 dots/inch(dpi). For electronic image data, the resolution can be defined as the physical resolution if the image would be rendered on a certain size.